As is known, in the aircraft industry, reducing noise, both external and inside the cabin, is becoming a major design issue.
Noise is mainly produced by the engines, the auxiliary components powered by the engines, moving parts, and airflow over the airframe, and propagates both directly to the outside, and into the aircraft itself, mainly along airflow and structural paths, i.e. the points connecting the cover panels to the fuselage.
Noise is reduced both by working directly on the noise-source components, and by applying noise-damping material between the structural frame of the helicopter and the cover panels, to minimize noise propagation from the outside to the inside of the cabin.